Where two or more vehicles, such as a fleet of buses or trucks, are connected electronically to one or more base stations, for purposes of periodically reporting operating data such as vehicle position and vehicle operating parameters to the base station, maintenance of a common accurate time for each vehicle and for the base station(s) is a concern. The vehicles may report their respective operating data by time sharing of access on a metropolitan area network, using an access allocation such as time division multiple access ("TDMA") or some other suitable access allocation approach. In time shared access, each vehicle in the network would be allocated one or more time slots in a given time interval to report its operating data to the base station(s). One or more additional time slots are optionally available (1) to allow the base station to poll the vehicle communications systems ("VCSs") for any additional information that should be communicated promptly to the base station and/or (2) to allow each VCS to attempt to communicate such information to the base station by contention during this additional time slot(s). This second time slot access mechanism is allocated using ALOHA, as discussed by M. Schwartz, Computer Communication Network Design and Analysis, Prentice Hall, 1977, pp. 286-320.
Several workers have disclosed time distribution systems using a master or base station and one or more subsidiary time signal receivers. An example is U.S. Pat. No. 3,520,128, issued to Novikov et al. An independent primary clock is connected to, and provides exact time signals for, a plurality of secondary clocks by radio waves. Each secondary clock receives a sequence of uncorrected "exact" time signals and a sequence of timing marks to correct this uncorrected time. The time signals for each secondary clock are apparently corrected separately.
Cater, in U.S. Pat. No. 3,811,265, discloses transmission of coded, time-indicating signals from a master clock at a central station to one or more slave clocks, using a two-wire line and binary-valued pulses with different time durations. A time synchronizing pulse is periodically inserted (e.g., once per second) on the line to correct for drift or other errors. If the two-wire line is a standard 60-cycle power line or a television cable, the binary-valued pulses use one or more frequencies that lie outside the frequency range normally used on that line, to avoid signal interference with the standard signals transmitted over that line.
A clock that can be synchronized by "wireless" signals is disclosed by Gerum et al in U.S. Pat. No. 3,881,310. The clock contains an electromagnetically operated mechanical oscillator whose frequency 2 f0 is twice the rated frequency of an alternating current network connected to the clock. A time synchronization module transmits a signal of frequency fl&gt;&gt;f0 that is modulated by the network at 2 f0 and received and demodulated using the clock. Normally, the pulses received from the network drive the clock and the oscillator is in a standby mode. The clock oscillator is enabled, and the network is disconnected, when and only when the network frequency differs by at least a predetermined amount from the frequency 2 f0 of the oscillator. The oscillator in standby mode receives resonance energy of frequency.apprxeq.2 f0 from the network for maintaining the oscillations.
U.S. Pat. No. 3,967,098, issued to Harnagel et al, discloses an air navigation instrument that combines calculator and timekeeping functions. An initial time is entered into an accumulator of the device, and the device computes and displays time in increments .DELTA.t=1 sec (or other chosen time interval) as the navigation calculations proceed. The time count can be captured and used in calculations at any time.
A TACAN air navigation system is disclosed in U.S. Pat. No. 3,969,616, issued to Mimken. Range of an aircraft from an interrogation signal-transmitting beacon is determined by the time elapsed between transmission of the interrogation signal and receipt of a reply pulse signal from the aircraft (called a "dwell" period in TACAN parlance). A circuit at the beacon generates and uses a filler pulse during any dwell period in which a reply pulse is not received from a target aircraft, in order to maintain an rough and unspecified synchronization at the beacon for the target aircraft when reply pulses are not received. An aircraft velocity detector may be included, with velocity being determined by averaging over several successive dwell periods to reduce the associated velocity error.
Cateora et al, in U.S. Pat. No. 4,014,166, disclose a satellite-controlled digital clock system for maintaining time synchronization. A coded message containing the present time and satellite position is transmitted from a ground station to an orbiting satellite and is relayed to a group of ground-based receivers. A local oscillator aboard the satellite is phase-locked to a precise frequency to provide the system with accurate time-of-year information by a count of the accumulated pulses produced by the oscillator. This count is compared with a time count determined from the coded message received by the satellite. After a selected number of errors are observed through such comparisons, the on-board clock is reset to the time indicated by the coded messages received. If transmission of the coded messages is interrupted, the on-board oscillator continues to provide time information that is transmitted to the ground-based receivers.
An antenna space diversity system for TDMA communication with a satellite is disclosed by U.S. Pat. No. 4,218,654, issued to Ogawa et al. Differences of temporal lengths of paths from the satellite through each antenna to a ground-based signal processor station are determined by measurement of times required for receipt of pre-transmission bursts sent in the respective allocated time slots through two different antennas, in a round trip from base station to satellite to base station. Variable time delays are then inserted in the base station signal processing circuits to compensate for the temporal length differences for the different signal paths. These time delays are changed as the satellite position changes relative to each of the antennas.
U.S. Pat. No. 4,287,597, issued to Paynter et al, discloses receipt of coded time and date signal from two geosynchronous satellites, which signals are then converted into local date and time and displayed. The frequency spectrum is scanned by an antenna to identify and receive the satellite signals. Temporal length differences for signal paths from each satellite through a receiving antenna to a signal processing base station are determined, to provide compensation at the base station for these differences. Time information is provided by a satellite every 0.5 seconds, and this information is corrected every 30 seconds. Signals from either or both satellites are used to provide the time and date information, in normal local time and/or daylight savings local time.
Jueneman discloses an open loop TDMA communications system for spacecraft in U.S. Pat. No. 4,292,683. A spacecraft, such as a satellite, in quasi-geosynchronous orbit carries a transponder that relays a coded signal from a ground-based signal-transmitting station to a plurality of spaced apart, ground-based receivers. This coded signal includes a time index and an index indicating the spacecraft's present position. The time index is adjusted by each receiver to compensate for the changing position of the spacecraft through which the coded signal is relayed. The system is open loop and requires no feedback from the receivers to the base station.
Nard et al, in U.S. Pat. No. 4,334,314, discloses a system for radio wave transmission of time-referenced signals between two ground-based stations, with compensation for multi-path transmission timing errors. Station no. 1 has a single antenna. Station no. 2 has two antennas, spaced apart by a selected distance, to allow measurement of and compensation for multi-path transmission path length differences. A signal processor located at the receiver antenna combines a plurality of timing marks, received from the transmitting antenna along multiple paths, into a single timing mark that compensates for the multiple path length differences. This arrangement allegedly allows station-to-station transmission over distances as large as ten times the trans-horizon or direct sighting distance (which is approximately proportional to the square root of the product of antenna height and Earth's radius).
Method and apparatus for determining the elapsed time between an initiating event and some other event are disclosed by U.S. Pat. No. 4,449,830, issued to Bulgier. A first timer and a second timer mark the times of occurrence, respectively, of an initiating event and a subsequent event that depends upon occurrence of the initiating event. The two timers are initially connected and synchronized, then disconnected before the initiating event occurs. The timers are then reconnected after both events have occurred, to allow determination of the elapsed time between occurrence of the two events.
In U.S. Pat. No. 4,482,255, Gygax et al disclose a timepiece for displaying both the present time and the present orientation of the time piece relative to the local Earth's magnetic field. The timepiece displays time, date, and the direction and angle through which the timepiece must be rotated in a tangent plane to align a fixed axis on the timepiece with the local field. The local magnetic field direction can be determined by two (static) Hall effect sensors placed at right angles to each other.
Distance ranging and time synchronization between a pair of satellites is disclosed by Schwartz in U.S. Pat. No. 4,494,211. Each satellite transmits a timing signal and receives a timing signal from the other satellite. The difference in time, including compensation for signal processing delay on a satellite, between transmission and receipt of the signals is transmitted by each satellite to the other satellite and is used to establish time synchronization and to determine the distance between the two satellites. This exchange of signals would be repeated at selected time intervals to maintain synchronization, where the satellites are moving relative to each other. No communications link to a third entity is required, and only one of the satellite clocks need be adjusted to establish and maintain time synchronization.
A portable timekeeping device that provides reminders (alarms) for taking certain actions at naturally occurring times is disclosed in U.S. Pat. No. 4,512,667, issued to Doulton et al. Means are provided for entering information on the present geographical location, and the device computes the appropriate times for taking the actions based upon the location and local time of day and year. The intended application here is for an alarm indicating the appropriate times after sunrise and before sunset for Moslem prayers. The present geographical location is entered and used together with the present time and present time of year (computed using a timekeeping device plus information stored in a ROM) to determine the appropriate times of day. A visually or audibly perceptible alarm is provided at each appropriate time of the day.
Plangger et al, in U.S. Pat. No. 4,582,434, disclose transmission and receipt of a continuously corrected single sequence of timing signals. A microprocessor at the receiver periodically compares these timing signals with on-board timing signals generated by a local clock. A varactor diode in a crystal oscillator circuit adjusts the microprocessor's operating frequency to minimize any error between the two timing signal sequences. Timing signal processing delay time is compensated for in a receiver circuit. The frequency for microprocessor operation is thus continuously corrected. If the transmitted timing signals are too weak or do not arrive, the on-board timing signals control the microprocessor until the transmitted timing signals are received in sufficient strength again.
Noguchi discloses a remote time calibration system using a satellite in U.S. Pat. No. 4,607,257. A base station provides a reference system of absolute timing signals and transmits these to a satellite that orbits the Earth. The satellite then calibrates and periodically adjusts its internally generated time and transmits observed data plus the corresponding adjusted satellite time to one or more data receiving stations on the Earth that are distinct from the base station. Time calibration optionally compensates for signal propagation time delay from base station to satellite and allows continuous transmission of data from satellite to the data receiving station(s). Several time difference indicia are computed here.
These approaches usually rely upon a single clock for time distribution. If signals from that clock are unavailable, or if the clock itself fails or is interrupted, no timing signal is available for the subsidiary systems. What is needed is a timing distribution system that provides a timing signal back-up that can be used whenever the primary timing signal is unavailable or, optionally, can be used intermittently to supplement the more accurate primary timing signal.